Reduction of nitrile to primary amine using sodium borohydride: Synthesis and protonation of a cis-octahedral macrocyclic nickel(II) complex bearing two 2-aminoethyl pendant arms

Abstract

The cis-octahedral complex [NiL2](ClO4)2·H2O (1) (L2=C-racemic-1,8-bis(N-2-aminoethyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane), bearing two coordinated N-CH2CH2NH2 pendant arms, has been prepared by a reaction of the N-CH2CN groups of [Ni(L1)(CH3COO)]+ (L1=C-racemic-1,8-bis(cyanomethyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) with NaBH4 in acetonitrile. The trigonal bipyramidal complex [Ni(HL2)](ClO4)3·H2O (2) (HL2=a mono-protonated form of L2), in which the primary amino group of the coordinated N-CH2CH2NH2 group occupies one of the three equatorial positions, has also been prepared by the addition of HClO4 to a methanol solution of 1. The equatorial Ni–N (N-CH2CH2NH2 group) distance (2.076(2)Å) of 2 is similar to other equatorial Ni–N (macrocyclic ring) distances (2.073(2) and 2.075(2)Å), but is significantly shorter than the Ni–N (N-CH2CH2NH2 groups) distances (2.170(3) and 2.190(3)Å) of 1. Interestingly, the coordinated N-CH2CH2NH2 group of 2 is inert to protonation.